The second generation cordless telephone (CT-2) Common Air Interface (CAI) describes the protocol for transmitting and receiving digitized audio and control data for second generation cordless telephones. A CT-2 handset receives an analog speech signal via a microphone, converts the analog speech signal into a digital speech signal, compresses the digital speech signal, modulates the compressed signal at a radio frequency, and transmits the modulated RF signal through an antenna. The transmitted RF signal is received by a nearby base station where it may be converted to an analog signal or remain in digital form for digital switching equipment. Ultimately, the signal is relayed to the destination telephone. When a similar signal is received from the destination telephone, the telephone signal undergoes the same process in reverse. The base station transmits a digital RF signal to the handset which is received via the antenna, demodulated, decompressed, and converted into an analog speech signal which drives a speaker in the handset.
The data is transmitted in one millisecond bursts every two milliseconds with each frame containing sixteen four-bit adaptive differential pulse code modulated (ADPCM) samples and either two or four bits of control data. Transmit and receive signals are sent and received to and from the base station in packets in a ping-pong fashion. CAI specifies that a part of the packet includes signalling information including such things as call setup and termination requests and handshaking information. For signal reception, a packet is received, demodulated, processed through a ADPCM decoder, converted from digital PCM to analog, and then provided to a speaker. In the CT-2 cordless telephone system, the handset establishes a digital link with the base station. The link is normally maintained until the call is completed.
Conventional CT-2 bases already have a telephone company (TELCO) interface for coupling the digital signal to digital switching equipment for relaying to the destination telephone. However, to sell a product having a TELCO interface requires stringent product testing and type approval from telecommunication standards setting entities.
Thus, what is needed is a CT-2 base suitable for use with a conventional telephone having a conventional TELCO interface already so that type approval is not necessary for the CT-2 base itself.